My question is about the use of multiple values for a single attribute.

When a note is under the responsibility of two encoders (for example #E1
and #E2), is it possible to
have <note resp="#E1 #E2">blablabla</note> ?

Is it then possible to express a kind of a boolean "OR" with the syntax
of the attributes ?
For example if, in a manuscript, the trancriber find it difficult to
elucidate the right "hand" out of a list of 2, but know it cannot be the
other hands found in the manuscript.
So the hand is #H1 OR #H2 but NOT #H3, #H4, #H5... How would you express
that ?

In general, is it possible to introduce set of values like for the rend
attribute as it exemplified below ? Or is it just used for the @rend
attribute ?
the example of the P5 guidelines : <head rend="align(center)
case(allcaps)">